During the operation of a generator in a wind turbine magnetic fields are induced from a rotor. The rotor contains permanent magnets or wound poles, which induce the magnetic fields into stator-cores and stator-coils. This leads to induced currents, which generate significant heat in the stator-cores and stator-coils.
Additional eddy currents contribute to the generation of heat. Eddy currents are generated when a conductor is exposed to a changing magnetic field due to a relative motion of the conductor and the magnetic field force. Eddy currents are also generated due to variations of the magnetic field over time.
The eddy currents create magnetic fields, which opposes a desired magnetic field between stator components and rotor components. This results in a eddy current loss.
The eddy current loss may reach a significant level, especially for a large electrical machine like a direct drive generator in a wind turbine. Thus the efficiency of the rotor is reduced.
Furthermore the heat, which is generated by the eddy currents, leads to an increased temperature in the stator-components.
A typical stator contains stacked laminate plates, which are made of metal. The laminate plates are punched out from a sheet of iron, for example.
FIG. 9 shows the shape of a known laminate plate LP, which is part of a stator in a generator.
A first side S1 of the laminate plate LP is aimed to an air gap, which is between the stator and a rotor of the generator. At this first side S1 there are a number of slots SL.
The slots SL are punched out from the laminate plate LP preferably.
A number of laminate plates LP will be stacked, so the slots SL form channels CH within the stack of laminate plates LP. The channels CH support metal windings MW of a stator coil.
Each metal winding MW is formed by a conductor CON, which is surrounded by a conductor-isolation CONI. Each slot SL shows a slot isolation lining SIL to insulate the metal windings MW.
There is a recess RC on each top of the slot SL. The recess RC is constructed to support a wedge WDG. By the wedge WDG the metal windings MW inserted are kept in place.
Heat is generated if the electrical machine is at work. The heat is generated mainly by the metallic windings MW of the stator. Due to the heat the isolation of the metallic winding MW may be damaged, thus the temperature of the winding needs to be cooled down to achieve a predetermined lifetime of the electrical machine.
Various arrangement and methods are known to cool large electrical machines. A very common one is the circulation of a gaseous medium like air inside the electrical machine. This gaseous medium is kept cool by a heat exchanger, for example.
The drawback of this method is that large gas-to-air or gas-to-water heat exchangers are needed. Furthermore additional power is needed to circulate the cooling medium.
Another common method is to circulate a liquid coolant on the stator-side, which is not adjacent or facing to the air gap. Thus heat is transferred from the metallic winding by conduction to the laminate plates and from the laminate plates by conduction to the cooling medium.
The drawback of this method is that a considerable temperature gradient will exist between the stator winding and the cooling medium due to the moderate heat conductivity of the laminate iron. Thus it is difficult to maintain the temperature of the winding below a required maximum value.
Another common method is to introduce a liquid or gaseous medium in hollow copper bars. These bars are installed below the windings of the stator coil or they are connected with the rear side of the stacked laminate plates by welding. The copper bars exceed the channels of the laminate plates.
The drawback of this method is that numerous joints are needed—for the electrical connection and for the connection of the hollow copper bars. Therefore this method is only used in very large generators.
Another drawback is that the cooling pipes are exposed to the same electromagnetic fields as the coil-windings, thus voltages will be induced in the cooling pipes, which are made of metal.
Another method is known from document US 2005 0067 904 A. Here the stator laminate, made of iron, comprises C-shaped slots on the stator side, which points away from the rotor. Cooling tubes are inserted in said slots and the tubes are deformed to fit into the C-shaped channels.
The drawback of this method is that the deformation of the cooling tubes may lead to small cracks in the tubes. These cracks may enlarge over time, for example due to corrosion, environmental influences or material characteristics. Thus the cracks will result in leaks later.
Another drawback of this method is that the length of the cooling tubes increases if the method is used in large electrical machines. In this case the cooling tube is shaped like a long “serpentine”. The cooling liquid is heated during its flow through the long serpentine. Thus the section of the stator, which contains the main part of the serpentine, will not be cooled sufficiently. Furthermore a temperature gradient will arise within the stator laminate, made of iron for example.